Why can't computers be random?
Computers (the processor in combination with the memory) are designed to be deterministic.  Otherwise no software would ever work, because the computer would be executing it randomly.

The computer's outcomes are not determined by the chaotic movements of electrons, but by the much strong deterministic currents and voltages between the transistors.  A digital machine is, simply speaking, not affected by noise.  The noise is always present,  but 0.13 volts or whatever gets rounded to 0 V (logic 0) and 3.02 volts or whatever gets rounded to 3.3 V (logic 1).

It is however possible to take advantage of the chaotic movement of electrons as well.  This is done in special hardware called "hardware random number generators", (some use different forms of true randomness such as nuclear decays).